Referring to FIG. 1, it shows a topology of a typical application circuit of a prior art flyback switching power supply 100 with high-voltage startup function, which mainly includes: a startup resistor R1, a startup capacitor C1, a switching circuit 101, a switching power supply control device 102, a flyback converter 103, a diode D1, a diode D2 and an output filter capacitor C2, wherein the switching power supply control device 102 may include an ON/OFF controller 105 and a PWM controller 106.
Referring to FIG. 2, it shows a waveform schematic diagram during the high-voltage startup of the flyback switching power supply 100, including waveforms at a power supply terminal VCC of the switching power supply control device 102, an output UV_CTRL of the ON/OFF controller 105, a high-voltage startup charging current Ich of the startup capacitor C1, a gate driving terminal GD of the switching power supply control device 102, and a DC output voltage VOUT.
In connection with FIG. 1 and FIG. 2, when the switching power supply 100 initiates the high-voltage startup, an AC input voltage VIN supplies a high-voltage startup charging current Ich to the startup capacitor C1 via the startup resistor R1, and a voltage at the power supply terminal VCC of the switching power supply control device 102 starts to rise. Meanwhile, the high-voltage startup charging current Ich increases from 0, reaches a maximum value of Ich0 (Ich0>0) and then decreases. When the voltage at the power supply terminal VCC is above a turn-on voltage VCCON of the ON/OFF controller 105, an output signal UV_CTRL of the ON/OFF controller 105 transitions from low to high, and the high-voltage startup charging current Ich decreases to Ich1 (Ich1>0), then the PWM controller 106 starts to work, generating a gate driving signal GD of the switching power supply control device 102. The gate driving signal GD may be a PWM modulated signal and controls the ON/OFF of the power transistor 104 in the switching circuit 101, such that a drain E_D of the power transistor 104 provides a power output current. Accordingly, the AC input voltage VIN delivers energy to the DC output voltage VOUT via a primary winding L1 and a secondary winding L2 of the transformer in the flyback converter 103, the diode D2, and the output filter capacitor C2, causing the voltage VOUT to rise. At the same time, the AC input voltage VIN supplies power to the power supply terminal VCC via the primary winding L1 and an auxiliary winding L3 of the transformer in the flyback converter 103, the diode D1, and the startup capacitor C1, in addition to supplying power to the power supply terminal VCC of the switching power supply control device 102 via the startup resistor R1 and the startup capacitor C1. As such, the switching power supply 100 completes the high-voltage startup, and begins to work.
In the above technique for high-voltage startup via the startup resistor R1, since the current Ich1 still flows through the startup resistor R1 after the startup, there is a tradeoff between the startup time and the standby power consumption. If the startup resistor R1 has a small resistance, during the startup, the AC input voltage VIN supplies a large current to charge the startup capacitor C1 via the startup resistor R1, resulting a short startup time of the switching power supply 100, but after the startup, the large current flowing through the startup resistor R1 causes high standby power consumption for the switching power supply 100. Otherwise, if the startup resistor R1 has a large resistance, during the startup, the AC input voltage VIN supplies a small current to charge the startup capacitor C1 via the startup resistor R1, resulting a long startup time of the switching power supply 100, but after the startup, the small current flowing through the startup resistor R1 causes low standby power consumption for the switching power supply 100.
To balance the startup time and the standby power consumption, in applications, the startup resistor R1 is generally chosen at a level of MΩ. Even so, when the AC input voltage VIN is at 220 VAC, the power consumption of the startup resistor R1 will be more than ten milliwatts (mW), up to hundreds of mW.
As above, the prior art switching power supply 100 which completes the high-voltage startup via the startup resistor R1 as shown in FIG. 1 can not both reduce the startup time, and decrease the standby power consumption.
Referring to FIG. 3, it shows a schematic diagram of a layout 201 of the power transistor 104 within the switching power supply 100 in FIG. 1. The power transistor 104 is a high-voltage MOS device.
In connection with FIG. 1 and FIG. 3, on the layout 201 of the high-voltage MOS device 104, a bonding pad for the gate G and a bonding pad for the source S are located on a front side, and a bonding pad of the drain D is located on a back side. The three bonding pads may function to provide a power driving output of the high-voltage MOS device 104.
Referring to FIG. 4, it shows a longitudinal cross-sectional view along a direction AA′ in FIG. 3.
As shown in FIG. 4, taking an N-type device as an example, the high-voltage MOS device includes: an N-type epitaxial region 306 of the MOS transistor, wherein the epitaxial region 306 is led out by an electrode 301 to form a drain of the MOS transistor; a P-well 302 of the MOS transistor; an N-type doped region 305 of the MOS transistor; a P-type doped region 309 of the MOS transistor, wherein the P-well 302, the P-type doped region 309 and the N-type doped region 305 are shorted via an electrode 303 to form a source of the MOS transistor; and a gate 304 of the MOS transistor. With regard to the whole structure of the device, the P-well 302, the N-type doped region 305, the P-type doped region 309 and the gate 304, etc., are formed in a cell portion 308. The cell portion 308 is a current conduction region of the device, and is an active region. The power device may be formed by a multitude of cell portions 308. There is a high-voltage ring 307 outside an edge of the cell portion 308. The high-voltage ring 307 may include a plurality of P-type doped regions 310, and may correspond to the region 207 shown in FIG. 3. The internal structure and the operational principle of the above device are well known in the art, and will not be described in detail.
In connection with FIG. 1 and FIG. 4, when a voltage applied to the gate 304 is higher than a threshold voltage, the surface of the P-well 302 is inversed to form a channel, such that the source and the drain of the MOS transistor are electrically connected to provide a power output.
In the scheme of FIG. 1, the high-voltage startup of the switching power supply 100 and the power supplying for the power supply terminal VCC of the switching power supply control device 102 are performed via the resistor R1. Since a current always flows through the resistor R1, there is a tradeoff between the startup time and the standby power consumption: if the resistance of the resistor R1 is small, during the high-voltage startup, a high current is supplied to the power supply terminal VCC via the resistor R1, resulting a short startup time of the switching power supply 100, but after the high-voltage startup, the large current flowing through the resistor R1 causes high standby power consumption for the switching power supply 100; otherwise, if the resistance of the resistor R1 is large, during the high-voltage startup, a low current is supplied to the power supply terminal VCC via the resistor R1, resulting a long startup time of the switching power supply 100, but after the high-voltage startup, the small current flowing through the resistor R1 causes low standby power consumption for the switching power supply 100.
To balance the startup time and the standby power consumption, in practice, the resistor R1 is generally chosen at a level of MΩ. Even so, when the input voltage VIN is at 220 VAC, the power consumption of the resistor R1 will be more than ten mW, up to hundreds of mW.
As above, the prior art switching power supply 100 which performs the high-voltage startup of the switching power supply 100 and power supply for the power supply terminal VCC of the switching power supply control device 101 via the resistor R1 cannot both reduce the startup time, and decrease the standby power consumption.
For the problem above, there is proposed a solution in the prior art to add a depletion-mode device for startup, as shown in FIG. 5. On the basis of the existing switching power supply, the switching power supply 400 in FIG. 5 adds a high-voltage startup device 403 to expedite the high-voltage startup procedure of switching power supply 400, wherein the high-voltage startup device 403 is a depletion-mode MOS transistor. After the high-voltage startup, the high-voltage startup device 403 is turned off to reduce the standby power consumption of the switching power supply 400, thereby improving the efficiency of the switching power supply 400.
In the prior art, the high-voltage startup device 403 is used as a discrete device, primarily for high-voltage signal processing and controlling. Since the high-voltage startup device 403 is a discrete device, the switching power supply 400 needs an extra component, which increases the complexity and cost of the system.